Depending on the field of use, electronic circuits are more or less subject to overvoltages on their inputs, which are caused by atmospheric discharges, for example. A typical example is the amplifying and signalling circuits in the line circuits for telephone exchanges, where the inputs and outputs can be subjected to overvoltages via the subscriber lines. These circuits are therefore provided with different forms of overvoltage protection which can be incorporated in the circuit itself, or adjacent it depending on the currents which must be led off.
If the electronic circuit is of the monolithic type then it can hardly withstand voltages in excess of the supplied voltage. At the same time it is desired that the circuit will cope with signal voltages in the vicinity of the supply voltage. It is therefore desirable that the potential of the input and output connections can vary within the normal voltage range of the source of current supply, but not appreciably outside this range.
A known method of protecting circuits for overvoltages in this way is to make each connection to the supply voltage terminals through limiting diodes, which are turned so that they are non-conducting as long as the connection potential falls below the potential of the voltage source positive terminal, but exceeds the potential of its negative terminal. The connection potential can therefore not exceed the positive terminal potential or fall below the negative terminal potential by more than one diode voltage drop. One terminal of the voltage source is grounded in many cases. In this circuit there will be an overvoltage in relation to ground, which is led off over a diode to the ungrounded terminal of the voltage source, to give a current which is not taken directly to ground but psses the voltage source. Depending on the internal impedance of the source, and the wave form of the overvoltage, unpermitted high voltages can therefore occur on the input in spite of the overvoltage guard.
A known possibility of avoiding this problem is to connect a diode, which is connected to the grounded terminal on the voltage source, in parallel with a varistor. The varistor is then dimensioned to lead such overvoltages to ground which cannot be taken away through the other diode and the internal impedance of the voltage source. In another known circuit, the varistor is replaced by the anode-cathode circuit in a thyristor. The thyristor gate is connected through a Zener diode to the grounded terminal of the voltage source. When the voltage difference between the connection and the grounded terminal is greater than the Zener voltage, the thyristor becomes conducting and the overvoltage is taken to ground. Both varistor and Zener diode must be selected with regard to the supply voltage for the guard to come into operation at the right level.